In industrial process measurement technology, particularly in connection with the automation of chemical or other industrial processes, analog or digital measured-value signals representative of process variables are generated using process meters installed on site, i.e., close to the process, so-called field meters. Examples of such process meters, which are familiar to those skilled in the art, are described in detail in EP-A 984 248, EP-A 1 158 289, U.S. Pat. No. 3,878,725, U.S. Pat. No. 4,308,754, U.S. Pat. No. 4,468,971, U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,574,328, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,617,607, U.S. Pat. No. 4,716,770, U.S. Pat. No. 4,768,384, U.S. Pat. No. 4,850,213, U.S. Pat. No. 5,052,230, U.S. Pat. No. 5,131,279, U.S. Pat. No. 5,231,884, U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,363,341, U.S. Pat. No. 5,469,748, U.S. Pat. No. 5,604,685, U.S. Pat. No. 5,687,100, U.S. Pat. No. 5,796,011, U.S. Pat. No. 6,006,609, U.S. Pat. No. 6,236,322, U.S. Pat. No. 6,352,000, U.S. Pat. No. 6,397,683, WO-A 88/02476, WO-A 88/02853, WO-A 95/16897, WO-A 00/36379, WO-A 00/14485, WO-A 01/02816, or WO-A 02/086426.
The respective process variables to be sensed may be, for instance, a mass flow rate, density, viscosity, a filling or threshold level, pressure, temperature, or the like of a liquid, powdery, vaporous, or gaseous process medium which is conducted or stored in a suitable process vessel, such as a pipe or tank.
To sense the respective process variables, the process meter comprises a suitable, generally physical-to-electrical, transducer which is mounted in a wall of the process-medium-storing container or connected into a process-medium-conducting line, and which provides at least one, particularly electric, measurement signal representing the sensed process variable as accurately as possible. The transducer is connected to suitable meter electronics, particularly to electronics designed to process or evaluate the at least one measurement signal.
Via a data transmission system coupled to the meter electronics, process meters of the kind described are linked together and/or to process control computers, where they send the measurement signal via (4- to 20-mA) current loops and/or digital data buses, for example. For the data transmission systems, Fieldbus systems, particularly serial systems, such as PROFIBUS-PA, FOUNDATION FIELDBUS, and the corresponding communications protocols are used. By means of the process control computers, the transmitted measurement signals can be further processed and visualized as corresponding measurement results, e.g. on monitors, and/or converted to control signals for process control elements, such as solenoid valves, electric motors, etc.
To house the meter electronics, process meters of the kind described comprise an electronics case which, as proposed in U.S. Pat. No. 6,397,683 or WO-A 00/36379, for example, may be located at a distance from the field meter and be connected to the latter by a cord, or which, as also shown in EP-A 903 651 or EP-A 1 008 836, for example, is disposed directly at the transducer or at a case that houses the transducer. Frequently, the electronics case, as shown in EP-A 984 248, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,716,770, or U.S. Pat. No. 6,352,000, for example, also houses some mechanical components of the transducer, such as diaphragm-, rod-, sleeve-shaped or tubular bodies which deform or vibrate under mechanical action.
Particularly in EP-A 1 158 289, U.S. Pat. No. 4,768,384, U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,687,100, WO-A 88/02476, WO-A 95/16897, or WO-A 01/02816, process meters for measuring at least one physical process variable, particularly a mass flow rate, density, viscosity, pressure, or the like, of a medium flowing in a process line are shown in which the respective transducers comprise:                at least one flow tube for conducting the, particularly flowing, medium;        an excitation assembly electrically connected to the meter electronics and comprising a, particularly electrodynamic or electromagnetic, vibration exciter for driving the flow tube; and        a sensor arrangement comprising at least a first sensor and a second sensor which respond to the physical process variable, particularly to changes therein, and provide at least a first measurement signal and a second measurement signal in response to the physical process variable,        wherein the meter electronics provide at least one excitation signal for controlling the vibration exciter, so that in operation, the flow tube is vibrated at least intermittently,        wherein the two sensors respond to inlet-side and outlet-side vibrations of the flow tube, respectively, and        wherein the measurement signals provided by the sensors represent mechanical vibrations of the vibrating flow tube which are influenced by the process medium.        
Moreover, such a vibratory process meter comprises a transducer case which encloses the flow tube along with the vibration exciters and sensors mounted thereon as well as any further components of the transducer.
If the vibratory process meter is used as a Coriolis mass flowmeter, the meter electronics determine, among other quantities, a phase difference between the two measurement signals from the sensors, here vibration signals, and provides at its output a measured-value signal which, corresponding to the temporal variation of the phase difference, represents a measured value of the mass flow rate.
As is well known, aside from the above-described process variables to be sensed, other physical quantities, particularly quantities that cannot be influenced, such as a process temperature or a process-medium temperature, may act on process meters of the kind described, particularly on their respective transducers.
Especially in process meters using vibrating flow tubes, for instance in Coriolis mass flowmeters, densimeters, and/or viscometers, thermal expansion of the flow tube may result in the transducer exhibiting, aside from a sensitivity to the primary measurands, such as flow rate, density, and/or viscosity, an unwanted sensitivity to an instantaneous temperature distribution in the transducer. Due to such spurious temperature effects on the oscillatory response of the transducer, the latter is “detuned”. Accordingly, if this “detuning” is not taken into account, the measured-value signal provided by the meter electronics may be erroneous.
To compensate for spurious temperature effects on the measurement signals provided by the transducer and/or on measured-value signals derived therefrom by the meter electronics, Coriolis mass flowmeters or Coriolis mass flowmeter-densimeters generally include at least one temperature sensor, for instance a sensor for measuring the temperature or ambient temperature of the flow tube, compare U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,687,100, or WO-A 88/02476.
In the process meters shown therein, in order to compensate for effects of temperature on the elasticity moduli of the respective flow tubes, an electric temperature measurement signal corresponding to the temperature of the medium being measured is produced by means of a temperature sensor mounted on a curved flow tube, for instance a Pt100 or Pt1000 element or a thermocouple. This temperature measurement signal is then multiplied in the meter electronics by constant, time-invariant coefficients to obtain a correction factor taking into account the influences of the measured temperature on the modulus of elasticity, and thus incorporated in the correction of the measured-value signal, e.g. a mass flow rate signal and/or a density signal. To smooth the temperature measurement signal or improve its signal-to-noise ratio, suitable digital signal filters may be employed, as proposed in WO-A 88/02476, for example.
Aside from such vibratory process meters with curved flow tube, vibratory process meters with a single straight flow tube or with two flow tubes are known to those skilled in the art, compare especially U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,768,384, U.S. Pat. No. 6,006,609, WO-A 00/14485, or WO-A 01/02816. In such process meters with a single straight flow tube, a supporting element fixed to the flow tube, particularly an element mounted in the transducer case so as to be capable of vibratory motion, is provided in the transducer for supporting the vibration exciter and the sensors. The supporting element also serves to vibration-isolate the vibrating flow tube from the connected pipe. It may be designed as a tubular compensation cylinder coaxial with the flow tube, or as a box-shaped support frame.
Because of their specific design, vibratory process meters with straight flow tube(s) respond to temperature changes not only with the aforementioned change in modulus of elasticity, but temperature-induced changes in mechanical stresses within the flow tube and within the supporting element and/or the transducer case also cause changes in the transducer's sensitivity to the primary process variables.
Such thermal stresses, particularly stresses acting along the axis of the flow tube, may have various causes, which may occur alone or in combination. Even if the flow tube and the supporting element or the transducer case are virtually at the same temperature, thermal stresses may occur if the supporting tube and the vibrating system are made of different materials with different coefficients of thermal expansion. Such effects of temperature on the measurement result are even stronger if the temperature of the flow tube is different from the temperature of the supporting tube. That is the case particularly if a process medium is to be measured whose temperature differs from the ambient temperature. In the case of very hot or very cold process media, a very high temperature gradient may exist between the supporting element or the transducer case and the flow tubes.
Measures to compensate for such temperature effects, which change the transducer's sensitivity to the primary process variables, are described, for example, in U.S. Pat. No. 4,768,384, U.S. Pat. No. 5,231,884, or WO-A 01/02816. Using at least one additional temperature sensor attached to the transducer case, the effect of thermal expansions of, or thermal stresses in, the transducer case on the measured-value signal are compensated for by forming in the meter electronics a further correction factor which takes into account the effects of the measured temperature on the expansions or the stress distribution in the transducer, and including this correction factor in the formation of the measured-value signal. To form this correction factor, each of the temperature signals multiplied, simultaneously and undelayed, by constant coefficients and, if necessary, by itself.
It turned out, however, that during the operation of process meters of the kind described, the temperature distribution may be subject to considerable variations, particularly because the temperature of the fluid cannot generally be held constant, so that within the process meter, particularly within the transducer, dynamic transients occur repeatedly with respect to the temperature distribution. On the other hand, these temporal variations in temperature distribution, because of different specific thermal conductivities or heat capacities of individual components of the transducer, may propagate to individual sensitivity-determining components of the transducer at different rates, so that even temperature profiles or temperature gradients sensed by means of two or more temperature sensors may be subject to dynamic changes.
In process meters which, as shown in U.S. Pat. No. 4,768,384 or WO-A 01/02816, for example, determine corresponding correction factors for the measurement signal using static algorithms which only take into account instantaneous temperature values, this may result in considerable inaccuracies occurring in the measured value signal during the unsteady state of the temperature distribution, despite the use of such correction factors derived from different temperatures which are sensed at different locations but uniformly weighted. Investigations have shown that such nonstationary transition regions of the temperature distribution, which particularly cause changes in the mechanical stresses within the transducer, may last from a few minutes to a few hours, and that during this frequently relatively long time of the unsteady state of the temperature distribution, the effects of the locally sensed temperatures on the measurement signal(s) in relation to each other may change as well.
In transducers with a vibrating flow tube, one possibility of reducing such errors in the measurement signal may be to install a plurality of temperature sensors along the flow tube and along the transducer case and/or along the supporting element that may be provided for the single flow tube.
One disadvantage of such a solution is that the manufacturing costs increase with the number of temperature sensors used. Aside from the costs of the temperature sensors themselves, the costs of mounting and wiring them increase.
In addition, however, an increase in the number of temperature sensors may result in an increased failure probability of the sensor arrangement, particularly if the temperature sensors are fixed to components vibrating at high frequencies, for instance to the flow tube or to a supporting element designed as an antivibrator.